Pyrithione (also known as 1-Hydroxy-2-pyridinethione, 2-pyridinethiol-1-oxide, 2-mercaptopyridine-N-oxide, pyridine-2-thione-N-oxide, pyridinethione-N-oxide, 2-pyridinethione, pyridinethione, or simply “PT”) has been noted for its bactericidal and fungicidal activities. Pyrithione is a bidentate ligand that forms stable complexes with most transitional metals. Metallization of pyrithione often results in highly augmented biocidial activities. Metal salts of pyrithione, such as for example, sodium pyrithione, magnesium pyrithione, barium pyrithione, bismuth pyrithione, strontium pyrithione, copper pyrithione, zinc pyrithione, cadmium pyrithione, and zirconium pyrithione, are widely used as fungicides and bactericides in a broad spectrum of commercial products, such as metalworking fluids, lubricants, paints, cosmetics and toiletries.
Zinc pyrithione (or “ZPT”) is especially useful as a broad-spectrum anti-microbial agent and preservative. It is active against both gram-positive and gram-negative bacteria, as well as fungi and yeasts. Therefore, ZPT has been used in various personal care compositions, such as for example, anti-dandruff shampoos, hair conditioners, leave-on tonics, and anti-microbial foot powders.
Bar soap is a popular product form for cleansing. A bar soap comprising ZPT is particularly desirable for its broad-spectrum antimicrobial efficacy. Aesthetics of consumer products such as bar soaps have significant impact on the consumers' perception of the products, which will in turn determine the acceptability of the products by the consumers. However, pyrithione-containing compounds can become discolored in the presence of ferric or cupric ions, even if the ferric irons are present only in trace amounts. The presence of such a color change is typically referred to as “discoloration” (usually to gray, green, blue or purple colors) and is believed to be due to the formation of a dark colored pyrithione precipitate compound from the reaction of the pyrithione groups with unwanted ferric (iron) or cupric ions that are found in the metal parts of the abovementioned manufacturing equipment. The metal ions can also be introduced into the soap compositions unintentionally as impurities in the raw materials used for making bar soap. Further, during manufacturing, handling or storage, various metallic parts of the manufacturing equipment, such as roller mills, pipes, or nozzles, may come into contact with the soap noodles or pellets, thereby introducing metal ions into the soap composition. In some situation, such contact can be maintained for a long time (e.g. overnight to 24 hours), and at a relatively elevated temperature, thereby increasing interaction between ZPT and metal ions.
The resultant discoloration may adversely affect the overall aesthetics of the bar soaps and give consumers a negative impression of the soap quality.
In the past, a number of solutions have been developed in attempt to solve the ZPT discoloration problem. For example, in U.S. Pat. Nos. 4,161,526 and 5,883,154, zinc salts (such as zinc acetate, zinc chloride and zinc sulfate), zinc hydroxide and zinc oxide have been used to address the ZPT discoloration problem. In U.S. Pat. Nos. 4,818,436, 4,957,658 and 4,533,736, 1-hydroxyethane-1,1-diphosphonic acid (HEDP), borate or sulfite has been used to reduce ZPT discoloration.
However, none of these solutions can completely eliminate or effectively reduce the undesirable discoloration in ZPT-containing bar soaps, which remains as a continuing concern for manufacturers, and more importantly, they can also lead to noticeable degradation of ZPT in the bar soaps. ZPT has been known to be unstable when solubilized. It may undergo transformation upon exposure to oxidizing species or certain metal cations, such as Cu2+ and Fe3+. The presence of the above-mentioned compounds in bar soap compositions further aggravates ZPT instability and results in significant ZPT loss over time, which in turn reduces the anti-microbial effect of ZPT-based bar soap compositions.
There is therefore a continuing need for improved ZPT-based anti-microbial bar soap products with improved discoloration resistance for more pleasing product aesthetics and better consumer appeal, as well as enhanced ZPT stability for extended shelf life of the products.